It is agreeing with the case of the subject, as nouns and adjectives do after verbs meaning be, appear, become, be thought, made, named, chosen, regarded, and the like (Pharr 974). That is to say, the thought of Agamemnon is being identified, in the strictest sense, with the subject. The case usage follows precisely the manner after which Homer has Achilles say in the first Book, "δειλός τε καὶ οὐτιδανὸς καλεοίμην, | εἰ δὴ σοὶ πᾶν ἕργον ὑπείξομαι".

The νόος of Agamemnon is not his mind, but what he has in his mind, the state of action and obedience he desires of his men; the lines do not well translate to what is just the English idiom, "For thou knowest not yet clearly what is the mind of the son of Atreus" (Murray), because then "mind" would be in the accusative as the direct object, as it is in lines 262-263 Book1. To this would be objected that the verbs are different: the example from Book1 uses the verb "to see", and perhaps the perfect "to know" just happens to adopt the predicate-identity usage as a peculiarity of Homeric grammar? I cannot say I know for sure, but I doubt this is the case, having respect for Homer and his logic. But I can say with assurance that the perfect meaning "to know" naturally follows as what has been seen; but the perfect after all is still just the perfect! The perfect describes states. Hence this interpretation: "Clearly you are not realizing the sort of Agamemnon's intention" = "you are not being the kind he wills (you to be)". This agrees better with the note Monro made regarding line 190: Odysseus conveys delicate exhortation; and he does so all the way through to the end of line 194, before finally bringing to mind consequences, appropriately and in a logical manner.

This interpretation though subtle is significant in appreciating the web Homer spins.

For some reason nobody noticed your post when you posted it. Nice that you came back nevertheless. I suppose it's because there used to be a lag in the moderation of the forum, but think that's fixed now.

The difference between the two examples is this, I think:

In the first example,οὐ γάρ πω σάφα οἶσθ’ οἷος νόος ‘Ατρεΐωνος·

οἷος νόος ‘Ατρεΐωνος is a full subordinate clause with νόος as the subject. In English, as in many other languages, the verb ("is", also called the copula) would be compulsory here, but in Greek you can leave it out, just like here. So you could also write this more fully:

οὐ γάρ πω σάφα οἶσθ’ οἷος νόος ἐστι ‘Ατρεΐωνος.

The translation "For thou knowest not yet clearly what is the mind of the son of Atreus" is right to the point (although "For thou knowest not yet clearly how is the mind of the son of Atreus" would be even more exact). Note that "is" is compulsory in English.

οἵον Πειρίθοόν τε Δρύαντά τε, ποιμένα λαῶν is not a subordinate clause, you could not supply ἐστι "is" or any other verb, as the second line just expands τοίους ... ἀνέρας of the preceding line, and hence they are in the same case (accusative). "I have never seen nor will I ever see such men, such as Peirithous and Dryas the shepherd of the people..." No place for "is" here.